// Problem: 
// Contest: 
// URL: 
// Memory Limit: 
// Time Limit: 
// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#include<bits/stdc++.h>
using namespace std;
#define int long long
inline int read(){
	int s=0,w=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=getchar();}
	while(ch>='0'&&ch<='9') s=s*10+ch-'0',ch=getchar();
	return s*w;
}
string s;
int z(int x,int y)
{
	int ret=0;
	while(x<y)
	{
		ret+=abs(s[x]-s[x+1]);
		x++;
	}
	return ret;
}
signed main()
{
	int n,k,ans=0;
	cin>>n>>k;
	cin>>s;
	int l=0,r=l+k-1;
	int f=z(l,r);
	for(int i=0;i<n-k+1;i++)
	{
		ans+=f;
		f=f-abs(s[l]-s[l+1])+abs(s[r]-s[r+1]);
		l++;
		r++;
	}
	cout<<ans;
	return 0;
}
